1. Field of the Invention
The present invention relates to an ink jet printing apparatus and more particularly to a construction of a print head used in a serial scan type ink jet printing apparatus capable of high quality printing and to a maintenance method thereof.
2. Description of the Related Art
The ink jet printing method is a system for transforming input image data into an output image using a liquid ink, so that a maintenance technology for a print head that ejects ink becomes very important. Major problems that require maintenance are briefly explained here.
(a) The print head generally has a plurality of nozzles to enhance a printing speed and resolution (unless otherwise specifically noted, the nozzle generally refers to an ink ejection opening, an ink path communicating with the ink ejection opening and an element for generating an energy to eject ink). Depending on input image data, there may occur nozzles that are not used for ink ejection during a printing operation. In such nozzles ink solvent evaporates from the ejection opening, increasing an ink viscosity in the ejection opening or the ink path. Thus, when these nozzles are applied a normal ink ejection energy to be activated again, they may fail to eject ink properly and stably.
(b) Ink droplets ejected from the nozzles during the printing operation include main ink droplets and fine ink droplets (also called mist), and the fine ink droplets may adhere to peripheries of the ink ejection openings of the print head. The adhering ink may in turn pulls the main ink droplets being ejected out of the nozzles, resulting in the ink droplets being deviated from an intended direction, i.e., degrading a projection linearity of the main ink droplets.
(c) If there are bubbles in an ink reservoir in the print head, i.e., in the ink path deep in the nozzle or a common ink chamber communicating with the ink path, a gas that has migrated through the material making up the nozzles and the print head may be trapped in the bubbles, thereby inflating them. The bubbles may also become inflated as the temperature increases during the printing operation. This prevents a smooth supply of ink from an ink source, resulting in an ink ejection failure.
To solve the above problems (a) to (c), the following maintenance techniques may be adopted.
(A) Depending on the time during which the ink ejection has not been performed and on the environment, a predetermined amount of ink is ejected to discharge viscous ink. This is done apart from the ink ejection operation that is performed to form an image on a print medium (this operation is referred to as a preliminary ejection).
(B) The number of times that the ink has been ejected from the nozzles is counted, and when the count value exceeds a predetermined value, a surface of the print head formed with ejection openings (referred to as an ejection face) is wiped by a wiping member (such as a blade) made of an elastic material such as rubber to remove the adhering ink from the ejection face (this operation is called a wiping).
(C) A suction force is applied to the ejection face as by a pump at a predetermined timing to suck out ink from the ejection openings to forcibly draw out ink from inside the ejection openings for the recovery of an ink ejection performance (this operation is referred to as a suction-based recovery operation). In a construction in which the print head and an ink tank as an ink source are separably combined so that the ink tank can be replaced, when the ink tank is separated for replacement, an ink supply system is open admitting a gas (air). Some apparatus perform the above-mentioned suction-based recovery operation after the ink tank is replaced, to remove the gas from the ink supply system.
Here, the wiping operation and the suction-based recovery operation will be explained briefly.
FIG. 1A and FIG. 1B show a print head as seen from its ejection face and side. Reference number 1101 represents a blade made of rubber to perform wiping, 1102 the ejection face, 1103 ejection openings, 1104 adhering ink, and 1105 a wiping direction. The wiping is an operation that, as shown in the figures, involves moving the blade 1101 in the direction of arrow 1105 while holding it in contact with the print head, thereby wiping the adhering ink 1104 off the ejection face 1102 by the blade.
The suction-based recovery operation on the other hand generally involves having a cap of rubber pressed against the ejection face 1102 to form a hermetically enclosed space therein and operating a suction pump communicating to the cap to generate a negative pressure to suck out ink from the ejection openings or nozzles 1103 of the print head into the cap so that the ink drawn out is discharged through an ink discharge tube connected to the suction pump.
The suction pump may be of a tube pump type which comprises a holding member formed with a curved surface along which to hold a flexible ink discharge tube, a roller capable of pressing the ink discharge tube against the holding member, and a rotatable roller support supporting the roller. That is, by rotating the roller support in a predetermined direction, the roller is pressed against and flattens the ink discharge tube as it rotates over the holding member. As a result, a negative pressure is created in the enclosed space in the cap, sucking out ink from the nozzles which is then drawn into the ink discharge tube and the suction pump. The ink is further moved toward an appropriate member (a waste ink holding member such as an absorbent).
In today's ink jet printing apparatus on which there are growing demands for higher print quality and speed, the number of inks used and the number of nozzles arrayed in the print head are significantly greater than those of several years ago. Thus, the maintenance technology for the print head assumes a growing importance.
Now, a current trend for higher image quality in the ink jet printing apparatus is briefly described.
Conventionally, in the ink jet printing apparatus a color reproduction is made basically by a subtractive color mixing using three primary colors, yellow, magenta and cyan. For an improved image quality, some printing apparatus use a black ink in addition to the three primary color inks to represent a high contrast; others use light inks with lower colorant contents (light cyan ink and light magenta ink) to improve color tone; and others introduce an ink droplet atomizing technology to reduce a granular impression of an output image.
For an even further improvement in image quality, a variety of technologies are being developed, such as one using special inks (color inks other than cyan, magenta and yellow inks) that cover a range of color or gamut that cannot be expressed by the above six color inks, or one using color pigment inks that make for an improved fastness of an output image, or one using a liquid that, when applied to a print medium before or after ink is applied, improves a glossiness.
One such ink jet printing apparatus for improving the image quality is available (for example, see Japanese Patent Application Laid-open No. 2001-138552) which, in addition to black, cyan, magenta, yellow, light cyan and light magenta inks, uses an orange ink lying at an intermediate angle area between yellow and magenta and a green ink lying between yellow and cyan to widen the color range that can be reproduced.